In October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. In doing so, the FCC allocated twenty-five (25) megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting, at which time twelve and one-half (12.5) MHz was owned by XM Satellite Radio, Inc. of Washington, D.C. and 12.5 MHz was owned by Sirius Satellite Radio, Inc. of New York City, N.Y. In 2007, both companies merged to form SiriusXM Radio of New York City, N.Y. and provide subscription-based digital audio that is transmitted from two different sets of communication satellites (XM and Sirius), and the services provided by these and other SDAR companies are capable of being transmitted to both mobile and fixed receivers on the ground.
Generally, in the XM satellite system, two (2) communication satellites were present in a geostationary orbit, wherein one satellite was positioned at longitude one hundred fifteen degrees (115) degrees (west), and the other at longitude eighty-five (85) degrees (east). Accordingly, the satellites were always positioned above the same spot on the earth. In the Sirius satellite system, however, three (3) communication satellites were present, which all traveled on the same orbital path, spaced approximately eight (8) hours from each other. Consequently, two (2) of the three (3) satellites were “visible” to receivers in the United States at all times. Since both satellite systems generally had difficulty providing data to mobile receivers in urban canyons and other high population density areas with limited line-of-sight satellite coverage, both systems utilized terrestrial repeaters as gap fillers to receive and re-broadcast the same data that was transmitted in the respective satellite systems.
In order to improve satellite coverage reliability and performance, SDAR systems generally use three (3) techniques that represent different kinds of redundancy known as diversity. The techniques include spatial diversity, time diversity, and frequency diversity. Spatial diversity refers to the use of two (2) satellites transmitting near-identical data from two (2) widely-spaced locations. Time diversity is implemented by introducing a time delay between otherwise identical data, and frequency diversity includes the transmission of data in different frequency bands. SDAR systems may utilize one (1), two (2), or all of the above-noted techniques.